How to write such Apache RewriteCond?

Question

Require:
When users visit http://ask.mysite.com, they are redirected to the maintenance page.
When accessing other URLs, such as http://bbs.mysite.com or http://mysite.com, it will display normally.

It is to determine the subdomain

Answer 1

RewriteCond %{HTTP_HOST} !^ask\.mysite\.com [NC]